In the field of electronic materials, the formation of wiring layers (conductive layers), the formation of conductive circuits, and the formation of electrodes and the like are all conducted using conductive pastes that contain conductive powders. Furthermore, conductive pastes are also used as conductive or heat conducting adhesives for bonding electronic components together. A conductive paste is prepared by adding binders, organic resins, and where necessary other additives, to a conductive powder containing silver powder, copper powder, aluminum powder, palladium powder, or powdered alloys thereof, and then mixing to a paste-like consistency.
A conductive paste requires a high level of conductivity, meaning the blend quantity of conductive powder must be high, but the paste should also exhibit a level of flowability and a low viscosity that enable ready production and use of the paste. For example, when a conductive paste is used to fill a through hole to effect an interlayer connection, because a high level of conductivity is required even though the hole is small, as much conductive paste as possible must be packed into the hole to enable the hole to be filled with no voids. However, a problem arises in that if the blend quantity of the conductive powder is increased in order to increase the conductivity, then the viscosity of the conductive paste increases, causing a deterioration in the hole filling characteristics.
The total quantity of binder required in a conductive paste is related to the packing density (the packing ratio) of the blended conductive powder. In other words, in those cases where the packing density of the conductive powder is low (the void ratio is large), first, a large quantity of binder is required to fill the voids between particles within the conductive powder, and then an additional quantity of binder must be added to ensure favorable flowability and a low viscosity. Accordingly, if the packing density of the conductive powder is low, then the powder ends up diluted with a large quantity of binder, meaning the quantity of conductive powder within the conductive paste cannot be increased.
In contrast, in those cases where the packing density of the conductive powder is high (the void ratio is small), the quantity of binder required to fill the voids between particles within the conductive powder is small, meaning a conductive paste with a high conductive powder content and a high level of conductivity can be obtained.
In order to raise the packing density of a powder, theoretically, equally sized spherical particles in which the particle size distribution curve exhibits a single peak should be used, and moreover, a combination of large and small spherical particles should be combined, so that the gaps between the large particles are filled with the small particles (see Powder Technology Handbook, first edition, first printing, February 1986 (pp. 101 to 107), edited by The Society of Powder Technology, Japan, published by Nikkan Kogyo Shimbun, Ltd.).
However, in commercially available conductive powders, the smaller the particle size, the more strongly the particles are aggregated together, and even if large and small particles are combined, the type of packing density thought to be theoretically possible cannot be achieved. For example, the relative packing density for a silver powder with a particle diameter of 5 to 20 μm is around 60% at most, whereas for a silver powder with a particle diameter of approximately 1 μm, the relative packing density is around 50% at most, and even if these powders are simply combined and mixed together, the relative packing density achieved is still no more than approximately 60%.